The present invention relates to particles of a rare earth phosphate having usefulness as a base material of rare earth phosphate-based phosphors mainly used in fluorescent lamps as well as a method for the preparation thereof. More particularly, the invention relates to phosphate particles of lanthanum, cerium and terbium in combination and a method for the preparation thereof.
As is well known by the disclosure, for example, in U.S. Pat. No. 3,634,282 and Japanese Patent Kokai No. 54-56086, lanthanum orthophosphate activated by cerium and terbium having a monoclinic crystal structure is useful as a phosphor in fluorescent lamps of the 3-wavelength type. This phosphor material is usually prepared from composite phosphate particles of lanthanum, cerium and terbium in combination, for which several wet-process methods have been proposed heretofore, for example, in Japanese Patent Publication No. 1-41673, Japanese Patent Kokai No. 4-388105, U.S. Pat. No. 3,507,804 and Japanese Patent Kokai No. 6-56412. The rare earth phosphate products prepared by the methods disclosed in the former two references, however, are disadvantageous due to the relatively high production costs because the phosphate particles obtained thereby consist of primary particles having a very small particle diameter smaller than 1 .mu.m so that the precipitates of the phosphate formed by the wet process are consolidated into hard cakes in the course of filtration and drying necessitating a pulverization or disintegration treatment before use as a base material of rare earth phosphate phosphors. On the other hand, the rare earth phosphate particles prepared by the method disclosed in the above mentioned third reference have a particle diameter as large as 10 .mu.m or even larger so that the phosphor prepared from such phosphate particles having a particle diameter of 10 .mu.m or larger is not suitable for use in fluorescent lamps for which the phosphor particles preferably should have a particle diameter of 1 to 10 .mu.m although such a phosphor consisting of so coarse particles can be used in cathode ray tubes. Further, the rare earth phosphate powder obtained by the method disclosed in the above mentioned fourth reference consists of particles having a particle diameter of 1 to 15 .mu.m but the particles are each an agglomerate of very fine primary particles having a particle diameter smaller than several hundreds of nm.
As is mentioned above, phosphors in general preferably consist of non-agglomerated discrete particles having a particle diameter of 1 to 10 .mu.m. Primary particles of a rare earth phosphate-based phosphor having a particle diameter smaller than the above mentioned range have a problem that the brightness of the phosphor as a luminescent material on the walls of fluorescent lamps is subject to decay in the lapse of time during service. When the phosphor particles have a relatively large particle diameter but consist of agglomerates of fine primary particles, on the other hand, the agglomerate particles are necessarily disintegrated during handling to produce fine dusty particles so that the yield of acceptable phosphor products is decreased greatly. Furthermore, the agglomerate particles obtained by the method disclosed in the above mentioned fourth reference have an irregular particle configuration so that difficulties are encountered that coating on the inner surface of a fluorescent lamp tube with a slurried coating composition of the phosphor can hardly be uniform enough when the phosphor material is a mixture of phosphor materials of different types as is the case in the phosphor composition for fluorescent lamps of the 3-wavelength type as compared with a coating composition of a phosphor material consisting of spherical or globular particles. In this fourth method for the preparation of a rare earth phosphate, namely, it is essential that the pH value of the reaction mixture during the precipitation reaction is kept constant by the addition of ammonia water or an alkali metal hydroxide sometimes in an amount of three times by moles based on the amount of the rare earth element so that serious problems are encountered relative to the production costs and in the environmental pollution relative to waste water disposal. An alternative method is proposed in Japanese Patent Kokai No. 4-130014 for the preparation of rare earth phosphate particles as a base material of phosphors having a particle diameter of 1 to10 .mu.m, in which an aqueous slurry of fine particles of a rare earth phosphate is dried by using a spray drier. This method of spray drying is also not free from the problem, like the other methods described above, that the particles are formed by agglomeration of fine primary particles of sub-micron order to form agglomerated particles of 1 to 10 .mu.m diameter which have low mechanical strengths and are readily disintegrated so that the workability in the processing thereof into a phosphor is poor and the yield of acceptable phosphor products is necessarily low.